Animals of phylum Chordata are fundamentally characterised by the presence of a notochord, a dorsal hollow nerve cord and paired pharyngeal gill slits. They are bilaterally symmetrical, triploblastic and coelomate, with an organ-system level of organisation, and they possess a post-anal tail and a closed circulatory system. Chordates differ from non-chordates in having a notochord, a dorsal hollow single central nervous system, gill slits in the pharynx, a ventral heart and a post-anal tail. The phylum is divided into three subphyla: Urochordata or Tunicata, Cephalochordata and Vertebrata. Urochordata and Cephalochordata are often called protochordates and are exclusively marine. In Urochordata the notochord is present only in the larval tail, while in Cephalochordata it extends from head to tail and persists throughout life. Examples are Ascidia, Salpa and Doliolum for Urochordata and Branchiostoma (Amphioxus or lancelet) for Cephalochordata. Members of Vertebrata possess a notochord during the embryonic period, which is replaced by a cartilaginous or bony vertebral column in the adult. Thus all vertebrates are chordates but all chordates are not vertebrates. Vertebrates also have a ventral muscular heart with two, three or four chambers, kidneys for excretion and osmoregulation, and paired appendages that may be fins or limbs.